Various depth-from-focus methods provide information about the 3D shape and location of an object by interferometry. Some of these methods are based on optical coherence, often referred to as coherence scanning interferometry (CSI). To measure the height of an object using CSI, the object or a reference surface is scanned relative to the other, such that the phase varies between the frequency components of light traveling along an optical path to and from the object and the frequency components of light traveling along an optical path to and from the reference surface. The scan position at which the optical path length difference between the two light beams is zero corresponds to a point where the components of the two beams are mutually in phase, resulting in an interference signal having maximum signal intensity. Since the position of maximum signal intensity varies depending on the height of the object structure being measured, it is possible to obtain a surface topography of the object by identifying the scan positions corresponding to maximum intensity at different locations on the object surface. To maximize the fringe contrast of detected interference signals, it is generally accepted that a well-designed interferometer should be balanced for refractive index dispersion, particularly when working with broadband light. An advantage of CSI is that it allows for measuring surface structures that are more than one half wavelength in surface height difference from one imaging pixel to the next, without the so-called fringe ambiguity characteristic of phase shifting interferometry (PSI).
However, given the relatively short coherence length of the interferometer system, it is often necessary to reposition one or more parts of the interferometer in order to identify the scan position corresponding to maximum intensity for structures having relatively tall features. Furthermore, it is often necessary to scan over many fringes of the interference signal before the position of maximum intensity can be identified.